may cause more than momentary or slight pain or distress and to provide a written narrative description of the methods and sources used to determine that alternatives to the procedure were not available. As noted in the Institutional Animal Care and Use Committee Guidebook (ARENA-OLAW, 2002), the animal-use protocol must provide sufficient information for the IACUC to evaluate the pain and/or distress potentially resulting from the study and the appropriateness of the methods proposed to minimize it. The attending veterinarian is an important team member, working with the researcher and the IACUC to ensure animal welfare when there is the potential for pain and/or distress.

Assessment of Pain

According to the Guide, “fundamental to the relief of pain in animals is the ability to recognize its clinical signs in specific species” (p. 64). Pain can be assessed by evaluating behavioral measures such as eating, socializing, and withdrawal reflexes, and physiologic measures such as heart rate and respiration rate (see Table 2-1). However, species, and even strains and individuals of the same species, may vary widely in their perception of and response to pain (NRC, 1992; Wixon, 1999). Even for an individual animal, pain sensitivity varies among different tissues and organs (Baumans et al., 1994), and pain sensitivities can be altered by pathologic processes or experimental procedures (Carstens and Moberg, 2000). For example, during the initial phase of lipopolysaccharide-induced fever, rats exhibit hyperalgesia, whereas they exhibit hypoalgesia during the later stages of the illness (Carstens and Moberg, 2000). The existence of these differences underscores the point that pain and distress exist as a continuum of experience. In addition, some animals may hide signs of pain; for example, it has been suggested that rats may mask pain during the dark-cycle hours to avoid displaying abnormal activity and increasing their risk of predation (Roughan and Flecknell, 2000).

AALAS (AALAS, 2000) suggests that the magnitude of the pain that the animal is expected to experience be categorized in the protocol and monitored and that there be an opportunity to adjust the pain category once the study is under way. It is important that researchers and animal-care staff have a solid knowledge of the normal and abnormal physiology, behavior, and appearance of the animals in their care (Anil et al., 2002; NRC, 1992).

Acceptable levels of noxious stimulation are those that are well tolerated and do not result in maladaptive behaviors. Acceptable levels range from an animal’s pain threshold to its pain tolerance level. Pain threshold is the stimulus level at which pain is first perceived, while pain tolerance is the highest intensity of painful stimulation that an animal will voluntarily accept. As the intensity of a stimulus approaches the pain tolerance level, an animal’s behavior will become dominated by attempts to avoid or escape the stimulus, and this degree of pain must be alleviated (Dubner, 1987).